Meninges: Structure and Function

Meninges

• Three connective tissue layers protect the brain and spinal cord: dura, arachnoid, and pia mater
• The primary function is to protect the brain and spinal cord from trauma by acting as a cushion
• Meninges contain and support the brain and spinal cord, preventing excessive movement
• The subarachnoid space, located between the arachnoid and pia mater, circulates cerebrospinal fluid (CSF)
• Meninges contribute to the blood-brain barrier, which controls substance passage to maintain a stable neural environment.
• The meninges contain immune cells for defense against infections and abnormalities

Meningeal Spaces

• The epidural, subdural, and subarachnoid spaces are located between the meningeal layers

Cranial Dura Mater

• Composed of dense irregular connective tissue
• Consists of the endosteal/periosteal layer that lines the skull's internal surface
• The meningeal layer covers the brain and forms folds that divide the cranial cavity

Major Cranial Dural Folds

• Falx Cerebri separates the two cerebral hemispheres
• Falx Cerebelli separates the two cerebellar hemispheres
• Tentorium Cerebelli separates the cerebellum from the posterior cerebral hemispheres

Spinal Dura Mater

• Continuous with the meningeal layer of the cranial dura mater
• Has only one layer

Epidural Space (Spinal Cord)

• Located between the vertebrae and dura mater
• Filled with fat to cushion the spinal cord

Epidural Space (Skull)

• A potential space that only becomes apparent in the presence of pathology

Cranial Arachnoid Mater

• Has finger-like projections called arachnoid villi, which project into the dural venous sinuses

Arachnoid Villi

• Act as one-way valves, draining CSF and maintaining normal pressure within the cranial cavity

Cranial Pia Mater

• The innermost, highly vascularized layer
• Subarachnoid space, containing CSF, is located between pia and arachnoid mater

Spinal Pia Mater

• Lateral extensions form denticulate ligaments, which anchor and protect the spinal cord

Functional Meninges

• Interference with CSF drainage causes CSF to accumulate in the ventricles, increasing pressure
• In babies, this increased pressure can cause the head to bulge due to soft fontanelles
• Untreated pressure can damage nervous tissue
• A shunt can be implanted to drain excess CSF into the superior vena cava
• In adults, fused skull bones prevent swelling, leading to rapid nervous tissue damage

Ventricular System

• An expanded tube that continues from the spinal cord's central canal into the brain.
• Contains four ventricles filled with cerebrospinal fluid (CSF).

Four Ventricles of the Brain

• Two lateral ventricles are the largest, located between the cerebral hemispheres and communicate with the third ventricle
• The third ventricle, between the two thalami, communicates with the fourth ventricle via the cerebral aqueduct
• The fourth ventricle, between the brainstem and cerebellum, connects to the subarachnoid space and central canal of the spinal cord

CSF Composition

• Primarily water, containing oxygen, glucose, proteins, and white blood cells
• Produced by the choroid plexuses in each ventricle, lined by ependyma

CSF Flow

• Flows from the ventricles through the subarachnoid space around the brain and spinal cord

Function of Ventricles and CSF

• CSF cushions the brain, absorbing shocks and impacts
• Brain is suspended in CSF, reducing its effective weight and preventing compression
• Transports nutrients and removes waste products
• Maintains a stable chemical environment for neural function, regulating ion concentrations
• Facilitates neural signalling by providing a medium for neurotransmitter diffusion

Brain Blood Supply

• Primarily supplied by internal carotid arteries (~80%) and vertebral arteries (~20%)
• Internal carotid arteries branch into the anterior and middle cerebral arteries

Anterior and Middle Cerebral Arteries

• Supply blood to the frontal, parietal, and temporal lobes, as well as parts of the diencephalon

Vertebral Arteries

• Originate from the subclavian arteries
• Merge to form the basilar artery, supplying the brainstem and cerebellum
• Bifurcates into posterior cerebral arteries

Posterior Cerebral Arteries

• Supply the occipital lobe, parts of the temporal lobe, midbrain and thalamus

Circle of Willis

• A network of interconnected arteries that provides collateral circulation
• Located at the base and anterior surface of the brain
• Includes seven arteries in the subarachnoid space

Dural Venous Sinuses

• Cerebral veins drain blood into dural venous sinuses, unlike veins in other body parts which run parallel to arteries

Key Venous Sinuses

• The confluence of sinuses and the transverse sinus receive drainage from other sinuses
• The sigmoid sinus leads to the internal jugular vein (IJV)

Internal Jugular Vein (IJV)

• Carries venous blood from the dural venous sinuses, exiting the skull and returning to the heart

Cerebral Cortex Layers

• Organized in six layers with distinct cellular compositions
• Contains special nerve cell types (small and large pyramidal cells, and stellate cells)

Molecular Layer (I)

• Contains few cell bodies
• A site for synaptic connections

External Granular Layer (II)

• Contains small pyramidal cells and stellate cells
• Processes sensory information and transmits it to deeper layers

External Pyramidal Layer (III)

• Contains medium-sized pyramidal neurons
• Projects to other cortical areas
• Processes sensory and motor information

Internal Granular Layer (IV)

• Rich in stellate cells
• Involved in sensory processing

Internal Pyramidal Layer (V)

• Contains large pyramidal neurons
• Projects to subcortical structures and the spinal cord
• Involved in motor control and output

Multiform Layer (VI)

• Composed of many neuron cell types
• Integrates information and sends feedback to lower brain regions

Clinical Significance of Cortex Layers

• Abnormal changes or disruptions can indicate neurological diseases, developmental disorders, or brain tumours

Cerebellar Cortex Layers

• Has three major histological layers
• Includes the molecular layer, Purkinje cell layer, and granular cell layer

Molecular Layer (Cerebellar)

• Contains dendrites of Purkinje cells, stellate and basket cells
• Integrates sensory input and modulates Purkinje cell activity

Purkinje Cell Layer

• Single layer of Purkinje cell bodies
• Fine-tunes motor coordination and motor learning

Granular Cell Layer

• Packed with granule cells
• Processes and integrates sensory information and generates motor commands

Major Parts of the Brain

• Vital for breathing, blood pressure, sensation, movement, thought, and emotion
• Consists of the cerebrum, cerebellum, and brainstem

Cerebrum

• Largest, most superior part of the brain
• Consists of left and right hemispheres divided by the longitudinal fissure

Cerebral Cortex

• The outermost layer of nerve cell tissue, covered by grey matter
• Grey matter is responsible for information processing

White Matter

• Mainly myelinated axons
• Connects and facilitates communication between different parts of the brain

White Matter Categories

• Commissures: Connect the left and right cerebral hemispheres (e.g., corpus callosum)
• Association Fibres: Connect different regions of the brain within the same hemisphere
• Project Fibres: Connect the cerebral cortex with other parts of the brain (e.g., internal capsule)

Cerebral Lobes

• Four major lobes visible from the outside: frontal, parietal, occipital, and temporal
• Insula and limbic lobes are also present

Frontal Lobe

• Involved in voluntary motor functions, concentration, verbal communication, decision making, planning, and personality

Parietal Lobe

• Involved in general sensory functions

Occipital Lobe

• Processes visual information and stores visual memories

Temporal Lobe

• Involved in hearing and smell

Insula

• Involved in awareness of body and emotional state, emotional responses, empathy, and taste interpretation

Limbic

• Involved in emotional, memory, and motivational processes

Sulci

• Shallow depressions between folds in the cerebrum
• Demarcate special regions and lobes

Gyri

• Folds and ridges on the cerebrum
• Increase the brain's surface area and contain nerve cell bodies and dendrites

Key Gyri

• Precentral Gyrus: Primary motor area controlling voluntary movements
• Postcentral Gyrus: Primary somatosensory area processing somatosensory information
• Superior Temporal Gyrus: Contains Wernicke's area for language interpretation
• Inferior Frontal Gyrus: Contains Broca's area for speech production

Homunculus

• Mapped representation of the body along the precentral and postcentral gyri
• Face, hands, and fingers have a greater cortical representation, allowing for fine motor skills and sensory input

Cerebellum

• Located posterior to the brainstem, beneath the occipital lobe
• Coordinates motor control, cognitive functions, balance, and coordination

Cerebellum Structure

• Contains approximately half of the brain's neurons despite occupying only 10% of its volume
• Has two hemispheres with ridges and grooves

Cerebellum Regions

• Vermis: Connects the two cerebellar hemispheres
• Flocculonodular Lobe: One of the three lobes of the cerebellum

Cerebellar Peduncles

• Superior: Connects the cerebellum to the midbrain
• Middle: Connects the cerebellum to the pons
• Inferior: Connects the cerebellum to the medulla oblongata

Cerebellar Fissures

• Primary Fissure: Separates the anterior and posterior lobes
• Horizontal Fissure: Separates the superior and inferior surfaces
• Posterolateral Fissure: Separates the inferior surface of the posterior lobe and the flocculonodular lobe

Cerebellum Structure Similarities with Cerebrum

• Outer layer of grey matter called the cerebellar cortex, folded into folia
• White matter deep to the grey matter called arbor vitae, with a tree-like appearance
• Masses of grey matter deep within the white matter called the cerebellar nuclei

Cerebellar Nuclei

• Dentate (largest), emboliform, globose, and fastigial
• Fine-tune motor coordination, balance, and motor learning

Dysfunction of Cerebellar Nuclei

• Can lead to cerebellar ataxia, an inability to coordinate muscle movement

Brainstem

• Connects the cerebral hemispheres with the spinal cord
• Includes the midbrain, pons, and medulla oblongata

Brainstem Function

• Relays sensory and motor signals between the brain and spinal cord
• Regulates basic life-sustaining functions such as breathing, heart rate, and consciousness

Midbrain

• Located above the pons
• Involved in sensory and motor functions, visual and auditory reflexes, eye movements, and the substantia nigra

Pons

• Located above the medulla oblongata
• Connects different regions of the brainstem
• Regulates sleep, respiration, and facial movements

Medulla Oblongata

• Located at the lowest part of the brainstem
• Controls essential functions like breathing, heart rate, blood pressure, and reflexes such as swallowing, coughing, and vomiting

Diencephalon

• Located between the cerebral hemispheres and the midbrain
• Plays a crucial role in sensory processing, homeostasis, and physiological regulation

Thalamus

• Acts as a relay station for sensory information, transmitting signals to the cerebral cortex
• Over 50 distinct nuclei relay sensory impulses, crude perceptions, motor functioning, autonomic activities, and consciousness

Hypothalamus

• Located below the thalamus
• Regulates temperature, thirst, hunger, and circadian rhythms
• Controls body’s hormones via the endocrine system
• Considered the homeostasis centre

Epithalamus

• Situated above and behind the thalamus
• Includes the pineal gland and habenula

Pineal Gland

• Secretes melatonin to promote sleepiness and regulate sleep-wake cycles

Habenula

• Connects to the limbic system and regulates mood, reward processing, stress responses, and olfaction

Basal Ganglia

• Interconnected nuclei found deep within the white matter of the cerebral hemispheres
• Regulates voluntary motor control, movement coordination, and cognitive functions

Basal Ganglia Dysfunction

• can cause movement disorders (Parkinson's, Huntington's, dystonia) and cognitive impairments

Limbic System

• Includes the limbic lobe, basal ganglia, thalamus, hypothalamus, amygdala, and hippocampus
• Structures are around the upper brain stem, corpus callosum, inner cerebellum, and floor of the diencephalon

Nervous System Functions

• Master controlling and communicating system of the body

Communication Method

• Cells communicate via electrical and chemical signals
• Responses are rapid, specific, and immediate

Sensory Input

• Sensory receptors detect internal and external stimuli
• Sensory afferent neurons carry information to the brain and spinal cord

Integration

• The nervous system processes and interprets sensory input
• Responses occur at conscious and unconscious levels

Motor Output

• Appropriate response caused by activating effector organs (muscles and glands)
• Motor efferent neurons carry information to effector organs

CNS

• Consists of the brain and spinal cord
• Protected by the skull and vertebral canal
• Command centre for integration and processing nervous information

CNS Includes

• Fibre tracts and nuclei
• Nuclei are collections of neuronal cell bodies in the grey matter
• Fibre tracts are bundles of axons in the white matter

PNS

• Includes parts of the nervous system outside the CNS
• Mainly consists of nerves and ganglia

PNS Nerves

• Bundles of axons extending from the brain (cranial nerves) and spinal cord (spinal nerves)
• 12 pairs of cranial nerves exit the skull foramina
• 31 pairs of spinal nerves exit the intervertebral foramina

PNS Function

• Links all parts of the body to the CNS
• Peripheral nerves form plexuses (networks of nerves)

PNS Ganglia

• Collections of neuronal cell bodies located outside the CNS

PNS Sensory Receptors

• Sensory receptors in the skin or photoreceptors in the retina of the eye

Sensory (Afferent) Nervous System

• Receives sensory information from sensory receptors and transmits to the CNS

Somatic Sensory Subdivision

• General senses: touch, pressure, vibration, temperature, proprioception
• Special senses: vision, hearing, balance, and smell

Autonomic (Visceral) Sensory Subdivision

• Transmits nerve impulses from blood vessels, viscera, and taste information to the CNS
• Detects chemical composition of blood, temperature, or stretch

Motor (Efferent) Nervous System

• Transmits motor impulses from the CNS to effectors (muscles or glands)

Somatic Motor Subdivision

• Conducts nerve impulses to skeletal muscles, causing contraction
• Under conscious control

Autonomic (Visceral) Motor Subdivision

• Innervates viscera, smooth muscle, cardiac muscle, and glands
• Not under conscious control

Functional Systems of the PNS

• Somatic Nervous System (SNS)
• Autonomic Nervous System (ANS)
• Enteric Nervous System (ENS)

SNS

• Voluntary nervous system
• Afferent and efferent components (skin, skeletal muscles, joints, all senses except taste)

ANS

• Involuntary nervous system
• Afferent and efferent components (smooth muscles, cardiac muscle, glands, and taste)

Sympathetic ANS

• Elevated activity and attention
• "Fight or Flight" response
• Increases blood pressure and heart rate

Parasympathetic ANS

• Opposite effect
• "Rest and Digest"
• Decreases blood pressure and heart rate

ENS

• Part of the ANS
• Nerve plexuses within the intestinal wall control digestive functions

Nerouns

• Main tissue is nervous tissue
• Highly cellular, cells tightly packed and intertwined

Nervous System Cell Types

• Neuroglia: Non-excitable, support and protect neurons
• Neurons: Basic structural unit, generate, transmit, and receive nerve impulses

Neuron Characteristics

• High Metabolic Rate: Requires continuous glucose and oxygen
• Extreme Longevity: Functional for many years
• Nonmitotic: Unable to divide and make new neurons
• Excitable: Respond to stimuli
• Exhibit Conductivity: Quickly propagate electrical charge along plasma membrane

Neuron Dendrites

• Main receptive or input regions, receives signals from other neurons
• Branch from cell body

Neuron Cell Body

• Enclosed by a plasma membrane and contains cytoplasm surrounding a nucleus
• Body contains typical organelles (e.g., mitochondria, ribosomes, and rough ER)
• Nuclei found in the CNS
• Ganglia found in the PNS

Neuron Axon

• Arises from the axon hillock and narrows
• Longer than dendrites
• Conducts impulses away from the cell body
• Many axons are covered with a myelin sheath

Neuron Types (Structural)

• Multipolar: Most common, all motor neurons and interneurons
• Unipolar: Most sensory neurons
• Bipolar: Found in special sense organs

Neuron Types (Functional)

• Sensory Neurons: Afferent, transmit impulses from receptors to the CNS
• Motor Neurons: Efferent, transmit impulses from the CNS to effectors
• Interneurons: Association neurons, located within the CNS and carry out integrative functions

Neuroglia (CNS Types)

• Microglial Cell: Defend against pathogens, remove debris, phagocytose wastes
• Astrocyte: Provide structural support, assist in neuronal development and repair, regulate synaptic transmission, regulate tissue fluid composition, and help with the formation of the blood-brain barrier
• Ependymal Cell: Assist in the production and circulation of cerebrospinal fluid
• Oligodendrocyte: Wrap around CNS axons. They myelinate and insulate CNS axons, thus allowing faster nerve impulse conduction.

Neuroglia (PNS Types)

• Satellite Cells: Protect and regulate nutrients for the cell bodies in ganglia
• Schwann Cells: Wraps around portions of PNS axons. They myelinate and insulate PNS axons, thus allowing for faster nerve impulse conduction.